In a radio communication system, it is preferable that the communication devices to be used in the system are small and the amount of energy consumption of the communication devices is low. Regarding this point, since a power amplifier which is provided in a communication device and which is used to amplify transmission signal power consumes a large amount of power, it is effective to improve the energy efficiency of the power amplifier. A power amplifier is generally used in an area where the energy efficiency with respect to the input transmission signal power is high. As power amplifiers of high energy efficiency, for example, a Doherty amplifier, an envelope elimination and restoration (EER) filter and/or the like are used. However, such power amplifiers may have input-output characteristics in which the relationship between the input power and the output power is distorted non-linearly. Then, when a power amplifier is used in an area where the input-output characteristics are non-linear, the waveform of a transmission signal that is output from the power amplifier may be deteriorated, and, as a result, unwanted high frequency components are produced in the transmission signal, and signal power may leak between neighboring channels.
Consequently, predistortion-type compensation techniques for compensating for the non-linear distortion by a power amplifier by obtaining inverse characteristics of the input-output characteristics which the power amplifier has, applying distortion in response to the inverse characteristics to a transmission signal and inputting the predistorted transmission signal to the power amplifier, have been proposed (for example, see Japanese Laid-Open Patent Publication No. 2005-333353, Japanese Laid-Open Patent Publication No. 2003-124752, Japanese Laid-Open Patent Publication No. 2010-50908, Hsin-Hung Chen, Chih-Hung Lin, Po-Chiun Huang, and Jiunn-Tsair Chen, “Joint Polynomial and Look-Up-Table Predistortion Power Amplifier Linearization,” IEEE Transactions On Circuits And Systems-II: EXPRESS BRIEFS, VOL. 53, NO. 8, August 2006, Y. Akaiwa, “Introduction to Digital Mobile Communication,” Wiley, New York (1997), Lei Ding et al., “A Robust Digital Baseband Predistorter Constructed Using Memory Polynomials,” IEEE Transaction On Communications, Vol. 52, No. 1, January 2004 and R. Marsalek, P. Jardin, and G. Baudoin, “From Post-Distortion to Pre-Distortion for Power Amplifier Linearization, “IEEE Communications Letters, VOL. 7, NO. 7, July 2003).
In the predistortion-type compensation techniques, part of a signal that is output from a power amplifier is fed back, and predistortion coefficients are updated such that the error between the output signal and a transmission signal, the distortion of which is compensated for using the predistortion coefficients, is minimized.
The signal that is output from the power amplifier is an analog signal, but the output signal that is fed back to learn the predistortion coefficients is converted to a digital signal by an analog-to-digital converter (hereinafter simply referred to as “A/D converter”). However, since an A/D converter samples an output signal with a specific sampling pitch, when the sampling frequency of the A/D converter is less than twice the maximum frequency of the output signal, aliasing is produced in the digitized output signal, and, as a result, out-of-band frequency components, which are frequency components not originally included in the output signal, are produced. In base a station that complies with Long Term Evolution (LTE), an output signal that is fed back may have a 500 MHz band. In such cases, to prevent aliasing from being produced, it is preferable to use a high-speed A/D converter having a sampling frequency of at least 1 GHz, preferably 2 GHz. However, such high-speed A/D converters are generally expensive.
To reduce the cost of A/D converters, study is in progress to alleviate the requirement of the sampling frequency of A/D converters by limiting the band of the feedback signal to be used to learn the predistortion coefficients. However, when an output signal that is fed back is input to an A/D converter having a comparatively low sampling frequency, out-of-band frequency components due to aliasing increase. Consequently, reducing the out-of-band frequency components by inputting an output signal that is fed back to an A/D converter after the output signal passes an anti-aliasing low-pass filter, has been proposed.